1. Field of the Invention
The present invention relates to a corrosion resistant aluminum heat exchanger, and more particularly, relates to a method for treating aluminum heat exchangers, especially automotive heat exchangers, to make them corrosion resistant.
2. Description of Related Art
Because of their excellent corrosion resistance, copper alloys have long been used for fabricating automotive heat exchangers, such as radiators, condensers and evaporators. In recent years, however, aluminum and aluminum alloys have begun to replace the copper alloys in automotive applications due to their generally lower cost and the potential for significant savings in weight.
Unfortunately, heat exchangers made of aluminum and aluminum alloys are more susceptible to corrosion. The aluminum heat exchangers are particularly susceptible to electrolytic corrosion caused by water-soluble salts which are widespread in the environment. For example, a heat exchanger used as the condenser of an automotive air conditioning apparatus will be exposed to extended operation at elevated temperatures while at the same time being exposed to the influence of water-soluble salts. These circumstances cause pits to form on the heat exchanger surface. Within a short period of time these pits quickly grow and eventually cause holes or cracks to form in the heat exchanger core leading to the leakage of refrigerant. Of course, in most cases the heat exchanger must then be replaced.
In order to prevent refrigerant leakage caused by such pit formation, the surface of automotive heat exchangers generally are chemically treated or are provided with a physical coating in order to form a protective film. Provided a faultless protective film can be formed and maintained on the surface of the heat exchanger, this treatment provides good protection against corrosion. However, it is virtually impossible to form a faultless protective film on the heat exchanger surface, thus satisfactory corrosion resistance is not attained by this method. Moreover, such films are easily damaged by physical impacts (e.g., from collisions, mishandling, etc.), causing cracks to form in the protective film thus destroying its integrity.
In another method designed to solve the corrosion problem, illustrated in FIG. 1, metals such as zinc (Zn) and/or tin (Sn) are incorporated into the fin material 2, in order to lower its electrical potential relative to that of flat tube 1. In this way, the fin material 2 is preferentially corroded, thus protecting the flat tube from corrosion. This approach, however, requires the use of special fin materials, thus increasing the expense of the heat exchanger.
It also is known to protect the heat exchanger by coating the heat exchanger core with a flux containing zinc chloride (ZnCl.sub.2), followed by brazing to diffuse the zinc over the surface of the heat exchanger. However, the flux itself is quite corrosive and if excess flux is left on the core it corrodes the tube wall leading to tube failure. Thus, in order to ensure that any excess flux is removed, the heat exchanger must be thoroughly washed after brazing. This procedure introduces another step in the manufacturing process and also requires expensive pollution abatement facilities to handle the wash water.
On the other hand, in cases where a noncorrosive flux is used for brazing the heat exchanger, while the washing of excess flux and problem with flux-related corrosion can be avoided, the above-described problems relating for example to electrolytic corrosion are still present and must be addressed.